The physical protection of, and accountability for, confidential materials is a matter of great concern in both the public and private sectors alike. There is a growing need for security seals which are simple to install yet provide a high degree of reliability. Additional constraints are imposed on security seal design by the requirement of nondestructive in-situ verification. Mechanical means for insuring the integrity of locked objects or enclosures have recently been supplanted by devices which exploit the advantages of fiber optic technology.
One prior art preassembled fiber optic security seal utilizes first and second connectors linked by a fiber optic bundle. One end of each optic fiber in the bundle is exposed through an opening in the first connector. The remaining end of each optic fiber is either exposed through a window in the second connector or wound around the hollow interior of the second connector and returned through the bundle to the opening in the first connector. The bundle is looped through a closure and the two connectors are snapped together to form the security seal. When light is thereafter passed through the opening in the first connector, the random intermixing of the optic fibers in the bundle produces a unique light output pattern or "fingerprint" from the fiber optic ends exposed in the window of the second connector and the fiber optic ends exposed through the opening of the first connector. Any tampering with the security seal after installation will disturb the individual optic fibers in the bundle, causing a different output pattern to appear than that observed at the time of installation.
U.S. Pat. No. 4,161,348 to Ulrich discloses a seal of this type wherein the use of an optic fiber bundle comprising spatially randomized optic fibers insures the generation of an essentially unique fingerprint for each individual seal arrangement. Although such seals have proven very effective, the reliance on visual fingerprint comparison during successive viewing intervals can detract from the reliability of the comparison operation. Indeed, the minute dimensions of the optic fibers in combination with the relatively large number of fibers involved often precludes accurate visual detection of small shifts in the output pattern. It is thus possible for a sophisticated intruder to violate the integrity of such seals without detection.
In an effort to overcome some of the disadvantages associated with the visual seal verification technique, security seal systems have been developed utilizing viewers of various types to sense the light pattern produced by the seal. A very effective reticle pattern viewer to measure the relative radial and polar coordinates of various light transmitting fiber ends in an illuminated fiber optic seal is disclosed in U.S. Pat. No. 4,095,872 to Stieff. Here the radial and polar coordinates are recorded for subsequent comparison purposes.
Alternately, a fiber optic seal may be inspected for signs of tampering by matching a positive or negative photograph of the seal fingerprint taken during the inspection with a negative or positive photograph of the seal fingerprint taken at the time of seal installation. Unfortunately, the measurement of the radial and polar coordinates of illuminated fiber ends, or the point by point comparison of photographic prints, is a time-consuming process normally requiring the use of a microscope, photographic equipment and other bulky equipment which severly limits applicability. This process is also subject to inaccuracy if there is misalignment between the viewer or camera and the fiber ends which create the seal fingerprint. Moreover, seal security may be surreptitiously compromised by inserting a photographic mask across that portion of the seal window observed during the illumination/verification process.
Microprocessor controlled fiber optic seal systems have been developed to enhance seal control and monitoring. In one known type of seal, a light source under the control of a microprocessor illuminates one end of the bundle with a time-ordered random number, while a photo-transistor circuit arranged at the remaining end of the bundle detects the random number in the form of light transmitted through the bundle. Any interruption of the light transmission or harrassment of the system circuitry clears the random number generator in the microprocessor to provide an indication of tampering. Systems of this type are illustrated in U.S. Pat. Nos. 4,106,849 and 4,130,341 to Stieff, and these systems eliminate the subjective uncertainties associated with earlier prior art fiber optic security seals. However, these systems do rely upon the transmission of a unique, confidential random number sequence to provide seal verification. If this random number sequence for a given seal can be ascertained, electronically or otherwise, the sequence can be duplicated in counterfeit fashion and used to defeat the integrity of the original seal.
There remains a need in the art for a truely reliable fiber optic seal which is simple to construct and install yet maintains a high degree of security.